The present invention relates to a signal transforming circuit, and more particularly to a signal transforming/combining circuit with far field cancellation.
The fast growing wireless market has created an urgent demand for smaller and cheaper handsets with increased functionality and performance. A major trend of circuit design is to incorporate as many circuit components into integrated circuit form as possible, whereby cost per wafer can be reduced. Inductors built in semiconductor wafers are widely used in CMOS based radio frequency (RF) circuits such as low-noise amplifiers, voltage-controlled oscillators, transformers, power combiners, and power amplifiers. An inductor is a passive electronic component that stores energy in the form of a magnetic field, and tends to resist any change in the amount of current flowing through it. When an inductor-based device, e.g., a power combiner, is implemented as a single-chip with other functional circuits, the inductor-based device may cause an interference problem. Specifically, if two inductor-based devices are installed in a single-chip transceiver, for example, at the same time, the inductor-based devices may produce undesired interaction due to various types of mutual coupling mechanisms. This may result in spurious receiver responses and unwanted frequencies in the transmission spectrum. The primary mutual coupling mechanism is usually the fundamental electromagnetic coupling between the two inductors in the two inductor-based devices respectively. In other words, to solve the interference problem, the two inductor-based devices in the single-chip should be isolated. Therefore, making a better isolation between two inductor-based devices in a single-chip to reduce the interference problem has become an important issue in the field of wireless communication systems.